The purpose of this project is a detailed investigation at the molecular level of the mechanism of action of vitamin D and its most biologically active metabolite 1,25-dihydroxy-vitamin D3(1,25-(OH)2-D3). The major thesis of the study is that in terms of its chemical structure and postulated mode of action the seco-steroid vitamin D (calciferol) is similar to that of other classical steroid hormones. The kidney is postulated to be the endocrine gland which produces in a physiologically regulated fashion small amounts of 1,25-(OH)2-D3. After systemic transport, this steroid interacts in the target tissues, the intestinal mucosa and bone to generate the usual biological response attributed to vitamin D. Also the biochemical and biological properties of other vitamin D metabolites and analogs, e.g. 25-hydroxy-vitamin D3, 24,25-dihydroxy-vitamin D3, 1 alpha-hydroxy-vitamin D3, 3-deoxy-1 alpha-hydroxy-vitamin D3 and 25-hydroxy-5,6-transvitamins D3 will be evaluated in this framework. The specific studies proposed include: (a) identification of the biochemical steps in the target intestinal mucosa as 1,25-(OH)2-D3 interacts with cytoplasmic, then nuclear receptors, to stimulate new genetic information required for stimulation of messenger RNA and protein biosynthesis (including calcium-binding-protein); (b) elucidation of the molecular aspects of the regulation of the renal production of 1,25-(OH)2-D3 via studies on the biochemical properties of this mitochondrial enzyme as well as changes in its rate of biosynthesis and biodegradation; (c) evaluation of the uptake of specific vitamin D metabolites by bone and/or cartilage tissue and identification of possible subcellular receptors as well as possible consequences on calcium and collagen metabolism; and (d) a detailed assessment of how changes in the chemical structure and conformation of the vitamin D steroid nucleus may be related to biological activity in specific target tissues so that the structure requirements of differentially acting D-analogs or an antivitamin may be identified.